Not usually. The shows for late seating are typically the first ones - before dinner, and the main seating shows are the late ones - after dinner. When they only have one show, then it is done later - after the late seating dinner is finished.

oh right hmmm. cos I thought that if we booked the later meals that we would have time to get back on the ship relax get showered etc before dinner at 8pm. But now I'll have to get the show in first right?!! am I defeating the purpose of booking the late meal when I'll need to be ready for say 6.30 anyway to see the show!

It should not be a problem to switch dinner seating, since you have a little time before the cruise. Contact whomever you booked the cruise through (TA or direct with the cruise line) and they should be able to switch your seating.
My wife and I prefer to have dinner at 6pm, and then do the shows, or whatever, after dinner. Gives the food time to settle, and, with the late seating, I would think that you'd be going to sleep really full. Assuming dinner is over at 9pm or so, that seems too late for me. Shows and events are usually done twice a night to accomodate the two dinner seatings.
Remember, this is a vacation! Wing it a bit-you'll like it!
Enjoy-we're on Liberty in Sept.,2009-can't wait!